Moving Rainbow Underlines

Programming - Mar 31, 2024

I absolutely love the design of the Sandwich site. Among many beautiful features are these headlines with rainbow underlines that move as you scroll. It’s not scroll-jacking — it’s just a minor design feature that uses scroll position to enact a little movement.

To draw the rainbows themselves, we could use a linear gradient with hard-stops, the same kinda concept as drawing stripes in CSS. That’s a big ol’ chunk of CSS, which is fine, but I see they’ve opted for a background-image instead. Here’s that as an SVG, which is 661 bytes (tiny tiny). We can make it look like an underline by setting the background-size to limit the height and position it along the bottom with background-position.

We’ll do it on an inline element so the underline breaks where the words break:

h1 {
  span {
    background-image: url(spectrum.svg);
    background-repeat: repeat-x;
    background-size: 100vw 0.2em;
    background-position: left bottom 5px; 

To animate it, we move the background-position-x. Not a particularly performant thing to animate, but we’re not really animating it anyway — it’s just moving based on scroll position. Rather than manually manipulate the background-position-x, we’ll set it with a custom property, then manipulate the custom property with JavaScript.

background-position-x: var(--scrollPos);

Updating that variable while the page scrolls is easy peezy:

window.addEventListener("scroll", e => {
  let scrollTop = document.body.scrollTop ? document.body.scrollTop : document.documentElement.scrollTop; 
  let newPos = scrollTop + "px";'--scrollPos', newPos);

Here it is working!

See the Pen
Rainbow Underlines
by Chris Coyier (@chriscoyier)
on CodePen.

See that kinda janky line where I’m either using document.body or document.documentElement? That’s a stupid cross-browser thing where the “scrolling element” is different in Safari versus everything else.

While doing this I learned that you can use document.scrollingElement instead to avoid the pain there. I’ll leave a comment in the code about that, but leave the original line for posterity.

Previous Next
We respect the property rights of others, and are always careful not to infringe on their rights, so authors and publishing houses have the right to demand that an article or book download link be removed from the site. If you find an article or book of yours and do not agree to the posting of a download link, or you have a suggestion or complaint, write to us through the Contact Us .
Read More